Custom Fields lets you add additional information on to your website that can be used by Posts, Pages or Custom Post Types.
One of the ways WordPress stores information is with meta data (data about data!).
By default WordPress provides us fields like Title, Categories, Tags, Featured Image, Excerpt and more. Once we enable Custom Fields, we can add almost any type of data we want.
You will define those new bits of information by giving them a name, which is known as meta key. You only have to create your Custom Field once, once you create it, it will be available for you on your website, and you'll be able to add the values or that specific information to it.